1. Field of the Invention
This invention relates to tubing dies that form molten plastic extrudate into thin walled plastic tubing having uniform wall thickness about the entire circumference of the tubing.
2. Description of the Prior Art
When plastic tubing is formed by extruding plastic through a die, the hub of the die forms the inner wall of the tubing and the outer housing of the die forms the outer wall of the tubing as the extrudate passes through the in-line die. In prior art tubing dies, a single passage was provided through one of the fins holding the die hub centered in the housing. When air entered into the hub of the die and thereafter, through a passage in the die into the interior of the tubing, the air passing through only one fin cooled that fin to a temperature well below the other fins holding the hub within the housing. As the plastic extrudate passed over the hub, around the fin that was being cooled, the wall of the tubing at that point became thinner than the wall adjacent the other fins because the plastic flowed more slowly around that fin. Accordingly, the thickness of the wall of the tubing was not uniform around its entire circumference.
In an effort to correct the situation resulting from one of the fins holding the hub being cooled by air entering through that particular fin, die designers created dies that had coils surrounding the die so that the air would enter into the coil and *be circulated around the outside of the die to be heated by the higher temperature of the die. The heated air was then directed into the hub through one of the several fins that hold the hub within the housing. While this method improved the manufacture of the thin walled plastic tubing, it was not uniformly successful since it required additional costs in building the die and also required sufficient turns of the coil to cause the air entering the hub through the single fin to be heated to a temperature approaching that of the plastic extrudate.
The present invention provides an in-line tubing die in which air enters the center hub of the die through all of the fins which hold the hub in position within the housing to thereby provide for uniform walls on the thin wall tubing being formed by the die.